The use of light for communications is known. Optical fiber communications are well developed and free space communications systems for example using infra-red diodes and photodiode sensors have been specified for example by the Infrared Data Association (IrDA) to communicate data between devices. Other free space light communication systems include light fidelity (Li-Fi) communications, which is a bidirectional high speed and fully networked wireless communication technology similar to Wi-Fi. It uses a form of visible light communication to complement radio frequency communication such as from Wi-Fi or cellular networks and can use visible light, infrared or near ultraviolet frequency spectrum rather than radio frequency spectrum to communicate data.
However light based communication systems operating in free space are typically low data rate systems. The power of the light sources is strictly limited and therefore the achievable signal to noise ratio is limited which in turn limits the modulation levels available.
With respect to FIGS. 1A and 1B typical light based communication systems are shown. FIG. 11A shows a first example wherein a customer device 100, such as a mobile phone or other electrical device, comprises a single photon avalanche diode (SPAD) Time of Flight (TOF) module 101. The SPAD TOF module 101 itself comprises a suitable transmitter element which may be any suitable light source. For example as described hereafter the light source is in the form of a Vertical Cavity Surface Emitting Laser (VCSEL) configured to generate an ‘uplink’ communications channel. However the light source may for example be any type of laser or light emitting diode. The SPAD TOF module 101 further comprises a single photon avalanche detection (SPAD) array which is configured to receive light and measure the intensity (or determine a count value which is proportional to the light intensity) of the received light and which can be used as the receiver for a ‘downlink’ channel. FIG. 1A furthermore shows a base station 103. The base station 103 may further comprise a light source 105 (which may be a further VCSEL) which operates as the transmitter for the downlink and a light sensor 107 (which may be a further SPAD) which operates as the receiver for the uplink. The base station 103 in some embodiments may be any suitable base or docking station and which may comprise a mechanical locating means for enabling the customer device 100 to be located relative to the base station 103 to optimize the communication light paths between the customer device 100 and the base station 103.
FIG. 1B shows a further example where a first customer device 100 (such as described with respect to FIG. 1A) and a first SPAD TOF module 101 is configured to communicate with a second customer device 110 which comprises a second SPAD TOF module 111. Communication between two customer devices may not produce as good a quality communications link as the docking station/base station system due to relative motion between the two devices.
However such communications systems have problems maintaining high data rates, for example data rates of the order of hundreds of megabits per second (˜100 Mps) are difficult to achieve.